Why would that be? If I transcribe the creatures affected into the text of the Theft of Life card it seems like I would:

"The first time this creature [the Gorgon] regenerates each round, prevent 1 point of healing, then heal your mage [my Necro] one damage"

then

"The first time this creature [my Necro] heals each round, prevent 1 point of healing, then heal your mage [his Wizard] 1 damage"

You are correct from what I can see. There are two copies of Theft of Life in play, the one on the Gorgon Archer activates when it tries to regenerate, and then the one that was stolen and placed on the Necro activates when the first ToL activates.